Rockets and missiles launched from or near the Earth's surface to deliver payloads into sub-orbital or orbital trajectories is are forms of space craft that employ rocket engines in one or more stages for necessary propulsion. More customarily referred to as spacecraft are the payloads which are lofted into space and are intended to remain in orbit for at least some predefined period, such as the familiar and successful Space Shuttle. The Space Shuttle requires very reliable rocket engines to perform attitude and velocity control maneuvers during launch, on-orbit and descent phases of space flight. For the protection of on-board personnel, payloads and the high value vehicle, those rocket engines are also required to have high margins of safety. To ensure the Space Shuttle's continuing success, much consideration has been given to upgrading its propulsion system. Some major purposes to an upgrade are to shift to rocket propellants that are non-toxic, such as liquid oxygen and ethanol, and to lower operating costs.
Such an upgrade requires new rocket engine designs. However, those new designs are found to raise formidable thermal issues: How to cool the rocket chambers during firing operations in both the pulsing or steady state modes of operation and, in respect of bipropellant rocket engines that react fuel and oxidizer to generate thrust, how to vaporize liquid oxygen prior to injection of the oxygen into the rocket engine to achieve reliable on-demand rocket bursts, particularly in the smaller size "vernier" rocket engines.
As those skilled in the art appreciate, some present rocket engines are cooled by radiating the heat generated in the engine into space. Such rocket engines are designed to operate at a temperature close to the melting point of the metals from which the rocket's combustion chamber is constructed, leaving a small thermal margin of safety. As higher operational temperatures are attained to gain greater engine performance from existing rocket engine designs, more exotic and expensive refractory materials are required for the combustion chamber construction. One such material is Columbium, which withstands continuous operating temperatures below 1350 degrees Centigrade. That metal is rare and difficult to work. Thus, obtaining higher rocket engine performance from systems relying principally on radiation cooling is inconsistent with the companion goal of reducing manufacturing costs.
As an alternative, fluid cooling is commonly employed in other in-flight rocket engines, such as the SCAT engine marketed by the present assignee, but those engines always use one of the available propellants as the coolant. This cooling technique is referred to as "regenerative cooling". In that approach, typically, the combustion chamber is maintained at acceptable temperatures by transferring heat to one propellant that is ducted externally along the engine's hot surfaces; the cooling propellant is thereby warmed and/or vaporized before being injected into the rocket engine's main combustion chamber for reaction with the other propellant. In such systems, it is much more difficult to provide large thermal margins for engine operation, since the cooling propellant cannot be discarded, the propellant's heat capacity is fixed and is generally less than that of water, and the available propellant flow rates and pressure are severely limited. The resulting high temperatures of the flight combustion chamber wall and the need for compatibility with the oxidizer or fuel require that the chamber and cooling paths be complex in design and fabricated of more expensive materials and fabrication techniques.
Water cooling of machinery and engines is not new and water cooling systems for such apparatus has long been known. Indeed recirculating cooling systems, using Freon instead of water, are used presently in the Space Shuttle active thermal control subsystem (ATCS) for thermal management of several other subsystems and payloads, during missions. A recirculating coolant system more likely familiar to the lay person is the cooling system found in automobiles, wherein coolant, a mixture of water and ethylene glycol ("anti-freeze"), is pumped from the auto's radiator, which is the coolant reservoir and heat exchanger, by the water pump into the engine block. The coolant flows through cooling channels in the automobile engine, contacting the hot metal, whereby, some of the engine's heat of combustion is conducted to the flowing coolant, and, heated, flows out the engine, through the thermostat, and returns to the radiator as hot coolant. The radiator fan and/or the auto's movement blows air against the radiator, transferring heat from the radiator fins to the ambient air, thereby cooling the coolant, as the coolant flows from the radiator's inlet to the radiator's outlet, where the coolant is again pumped back into the engine. As one appreciates, a water cooling apparatus takes up additional space and adds weight in comparison to air cooling systems or radiant cooling systems.
Because of its con venience and effectiveness on the ground, water has also been used to cool rocket engine prototypes in on-the-ground testing throughout the history of liquid rocket propulsion. Water has one of the highest known specific heat capacities of any fluid, is inexpensive and is in plentiful supply. With its high specific heat capacity, water can be used to remove very large amounts of heat from combustion chamber walls.
With a plentiful supply of inexpensive water available at the test site, such as supplied by a municipal water company, the water main's spigot is turned on and fresh water flows into the test plumbing and is routed along the rocket combustion chamber walls. In the conductive thermal exchange relationship with the rocket engine, the flowing water draws heat away from the engine. The heated water flows from the rocket engine and into a drain, where it's simply discarded. Overall, the procedure is no more complicated than opening the water tap to obtain a drink of water, and pouring the unconsumed water down the drain. As those skilled in the art appreciate, the foregoing ground test cooling system is essentially an "open loop" cooling system.
When evaluating experimental rocket engine designs, because of the easy availability of such an effective coolant source on the ground, the prototype rocket engines can be equipped with combustion chamber walls that are formed of very inexpensive and easily shaped materials, such as steel or copper, whereby proving the design is accomplished at a relatively low cost. However, when the design is proven and finalized and a rocket engine is built for flight use, the more exotic metals, capable of withstanding the high temperatures encountered for radiation cooling of the rocket engine, are substituted for the copper and steel components. It has thus been easy to provide large thermal margins in ground tested rockets by proper selection of heat transfer geometry, fluid flow velocity and operating pressure.
The open loop cooling system for prototype testing is seen as simple, effective and inexpensive, and offers no incentive to discover a more exotic cooling system or better coolant for the test application.
Thus, a principal object of the present invention is to improve upon the safety margin, operational reliability, operational life and performance of space craft rocket engines and concurrently reduce the engines construction cost.
A further object of the invention is to provide a space craft propulsion system with a new cooling system that minimizes reliance on radiant cooling.
And a still further object of the invention is to provide a rocket engine cooling technique for in-flight use that employs a separate non-propellant fluid as a coolant, the non-propellant fluid having a significantly greater heat capacity than propellant fluids, in which the non-propellant fluid is reused and not discarded.